Electrical and Optical Modelling of Thin-Film Silicon Solar Cells
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0989-A03-01
Electrical and Optical Modelling of Thin-Film Silicon Solar Cells Miro Zeman1, and Janez Krc2 1 ECTM/DIMES, Delft University of Technology, Feldmannweg 17, 2628 CT Delft, Netherlands 2 Lab. of Photovoltaics and Optoelectronics, University of Ljubljana, Trzaska 25, 1000 Ljubljana, Slovenia
ABSTRACT Today amorphous and microcrystalline silicon based solar cells use surface-textured substrates for enhancing the light absorption and buffer and graded layers in order to improve the overall performance of the cells. Tandem and triple-junction configurations are utilized to assure better use of the solar spectrum and, thus, achieve higher conversion efficiencies of the devices. Resulting structures of the solar cells are complex and computer modeling has become an essential tool for a detailed understanding and further optimization of their optical and electrical behavior. The performance limits of tandem and triple-junction silicon based solar cells are studied by simulations using the optical simulator SunShine developed at Ljubljana University and the optoelectrical simulator ASA developed at Delft University of Technology. First, both simulators were calibrated with realistic optical and electrical parameters. Then, they were used to study the required scattering properties, absorption in non-active layers, antireflective coatings, the crucial role of the wavelength selective intermediate reflector, and a careful current matching in order to indicate the way for achieving a high photocurrent, more than 15 mA/cm2 for a tandem aSi:H/µc-Si:H and 11 mA/cm2 for a triple-junction a-Si:H/a-SiGe:H/µc-Si:H solar cells. By optimizing electrical properties of the layers and interfaces, for example using a p-doped a-SiC layer with a larger band gap (EG > 2 eV) and introducing buffer layers at p/i interfaces, the extraction of the charge carriers, the open-circuit voltage and the fill factor of the solar cells are improved. The potential for achieving the conversion efficiency over 15% for the a-Si:H/µc-Si:H and 17 % for the triple-junction a-Si:H/a-SiGe:H/µc-Si:H solar cells is demonstrated. INTRODUCTION Numerical simulation of optical and electrical behavior of semiconductor devices has been world-wide established as an essential tool for the improvement of existing devices, obtaining insight into their physical operation and for the development of new ones. A number of sophisticated semiconductor device simulation packages are already commercially available on the market such as Atlas from SILVACO company [1], Apsys from Crosslight company [2] and Taurus Medici from Synopsis company [3]. These programs are mostly designed for twodimensional (2-D) modeling of a broad range of crystalline semiconductor devices, but they are gradually updated to offer possibilities to model polycrystalline and amorphous semiconductor based devices such as thin-film transistors and solar cells. The advantage of these programs is that they are modular, so the users need to acquire only the minimum set of modules to meet their needs.
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